# Integration Period

DiFX accommodates a nearly continuous range of correlator integration periods over the range of practical interest. *Individual* integrations are quantized in multiples of the indivisible internal FFT interval, which is equal to the number of spectral points requested, divided by the sub-band bandwidth.

For most cases, with low to moderate spectral resolution, and/or wide sub-bands, the FFT intervals are fairly short, and it is straightforward to find an integration period in any desired range that is an optimal integral multiple of the FFT interval. ("Optimal'' refers here to the performance of DiFX.) Extreme cases of very high spectral resolution (many spectral points across a narrow sub-band - resolution of less than about 100 Hz) imply FFT intervals long enough that only limited choices of integral multiples are available.

For flexibility in these situations (although the option exists in all cases), integration periods other than an integral multiple of the FFT interval can be approximated, in a long-term mean, by an appropriate sequence of nearby optimal integral multiples. In this case, output records are time-tagged as if correlated with exactly the requested period.

SCHED now accepts an additional parameter so that users can indicate that the requested integration period is to be implemented exactly, as described above. Otherwise, the nearest optimal integral multiple of the FFT interval is passed to the correlator.